High-power Converters and AC Drives: Intro

This section provides an overview of high-power converters and medium-voltage
(MV) drives, including market analysis, drive system configurations, power
converter topologies, drive product analysis, and major manufacturers.
The technical requirements and challenges for the MV drive are also summarized.
These requirements and challenges will be addressed in the subsequent sections,
where various power converters and MV drive systems are analyzed.

INTRODUCTION

The development of high-power converters and medium-voltage (MV) drives
started in the mid-1980s when 4500-V gate turn off (GTO) thyristors became
commercially available. The GTO was the standard for the MV drive until
the advent of high-power insulated gate bipolar transistors (IGBTs) and gate commutated thyristors (GCTs) in the late 1990s. These switching devices
have rapidly progressed into the main areas of high-power electronics due
to their superior switching characteristics, reduced power losses, ease
of gate control, and snubberless operation.

The MV drives cover power ratings from 0.4 MW to 40 MW at the medium voltage
level of 2.3 kV to 13.8 kV. The power rating can be extended to 100 MW,
where synchronous motor drives with load commutated inverters are often
used.

However, the majority of the installed MV drives are in the 1- to 4-MW
range with voltage ratings from 3.3 kV to 6.6 kV as illustrated in Ill.
1-1.

The high-power MV drives have found widespread applications in industry.

They are used for pipeline pumps in the petrochemical industry [5], fans
in the cement industry, pumps in water pumping stations, traction applications
in the transportation industry, steel rolling mills in the metals industry, and other applications. Since the beginning of the 21st century a few thousands
of MV drives have been commissioned worldwide. Market research has shown
that around 85% of the total installed drives are for pumps, fans, compressors and conveyors, where the drive system may not require high dynamic performance.
As shown in Ill. 1-2, only 15% of the installed drives are nonstandard
drives.

One of the major markets for the MV drive is for retrofit applications.
It is re ported that 97% of the currently installed MV motors operate at
a fixed speed and only 3% of them are controlled by variable-speed drives.
When fans or pumps are driven by a fixed-speed motor, the control of air
or liquid flow is normally achieved by conventional mechanical methods,
such as throttling control, inlet dampers, and flow control valves, resulting
in a substantial amount of energy loss.

ill. 1-1 Voltage and power ranges of the MV drive. Source: Rockwell Automation.

The installation of the MV drive can lead to a significant savings on
energy cost. It was reported that the use of the variable-speed MV drive
resulted in a payback time of the investment from one to two and a half
years [7]. The use of the MV drive can also increase productivity in some
applications. A case was reported from a cement plant where the speed of
a large fan was made adjustable by an MV drive. The collected dust
on the fan blades operated at a fixed speed had to be cleaned regularly,
leading to a significant downtime per year for maintenance. With variable-speed
operation, the blades only had to be cleaned at the standstill of the production
once a year. The increase in productivity together with the energy savings
resulted in a payback time of the investment within six months.

ill. 1-3 shows a general block diagram of the MV drive. Depending on the
system requirements and the type of the converters employed, the line- and motor side filters are optional. A phase shifting transformer with
multiple secondary windings is often used mainly for the reduction of line
current distortion.

The rectifier converts the utility supply voltage to a dc voltage with
a fixed or adjustable magnitude. The commonly used rectifier topologies
include multi-pulse diode rectifiers, multi-pulse SCR rectifiers, or pulse-width-modulated
(PWM) rectifiers. The dc filter can simply be a capacitor that provides
a stiff dc voltage in volt age source drives or an inductor that smoothes
the dc current in current source drives.

The inverter can be generally classified into voltage source inverter
(VSI) and current source inverter (CSI). The VSI converts the dc voltage
to a three-phase ac voltage with adjustable magnitude and frequency whereas
the CSI converts the dc current to an adjustable three-phase ac current.
A variety of inverter topologies have been developed for the MV drive,
most of which will be analyzed in this guide.

TECHNICAL REQUIREMENTS AND CHALLENGES

The technical requirements and challenges for the MV drive differ in
many aspects from those for the low-voltage (_ 600 V) ac drives. Some of
them that must be ad dressed in the MV drive may not even be an issue for
the low-voltage drives. These requirements and challenges can be generally
divided into four groups: the requirements related to the power quality
of line-side converters, the challenges associated with the design of motor-side
converters, the constraints of the switching devices, and the drive system
requirements.

Line-Side Requirements

(a) Line Current Distortion. The rectifier normally draws distorted line
cur rent from the utility supply, and it also causes notches in voltage
waveforms. The distorted current and voltage waveforms can cause numerous
problems such as nuisance tripping of computer-controlled industrial processes,
overheating of transformers, equipment failure, computer data loss, and malfunction of communications equipment. Nuisance tripping of industrial
assembly lines often leads to expensive downtime and ruined product. There
exist certain guidelines for harmonic regulation. The rectifier used in
the MV drive should comply with these guidelines.

(b) Input Power Factor. High input power factor is a general requirement
for all electric equipment. Most of the electric utility companies require
their customers to have a power factor of 0.9 or above to avoid penalties.
This requirement is especially important for the MV drive due to its high
power rating.

(c) LC Resonance Suppression. For the MV drives using line-side capacitors
for current THD reduction or power factor compensation, the capacitors
form LC resonant circuits with the line inductance of the system. The LC
resonant modes may be excited by the harmonic voltages in the utility supply
or harmonic currents produced by the rectifier. Since the utility supply
at the medium voltage level normally has very low line resistance, the
lightly damped LC resonances may cause severe oscillations or overvoltages
that may destroy the switching devices and other components in the rectifier
circuits. The LC resonance issue should be addressed when the drive system
is designed.

Motor-Side Challenges

(a) dv/dt and Wave Reflections. Fast switching speed of the semiconductor
devices results in high dv/dt at the rising and falling edges of the inverter
output voltage waveform. Depending on the magnitude of the inverter dc
bus volt age and speed of the switching device, the dv/dt can well exceed
10,000 V/_s.

The high dv/dt in the inverter output voltage can cause premature failure
of the motor winding insulation due to partial discharges. It induces rotor
shaft voltages through stray capacitances between the stator and rotor.
The shaft voltage produces a current flowing into the shaft bearing, leading
to early bearing failure. The high dv/dt also causes electromagnetic emission
in the cables connecting the mo tor to the inverter, affecting the operation
of nearby sensitive electronic equipment.

To make the matter worse, the high dv/dt may cause a voltage doubling
effect at the rising and falling edges of the motor voltage waveform due
to wave reflections in long cables. The reflections are caused by the mismatch
between the wave impedance of the cable and the impedances at its inverter and motor ends, and they can double the voltage on the motor terminals
at each switching transient if the cable length exceeds a certain limit.
The critical cable length for 500 V/_s is in the 100-m range, for 1000
V/_s in the 50-m range, and for 10,000 V/_s in the 5-m range.

If not mitigated, they will appear on the neutral of the stator winding
with respect to ground, which should be zero when the motor is powered
by a three-phase balanced utility supply. Furthermore, the motor line-to-ground
voltage, which should be equal to the motor line-to-neutral (phase) voltage,
can be substantially increased due to the common-mode voltages, leading
to the premature failure of the motor winding insulation system. As a consequence,
the motor life expectancy is shortened.

It is worth noting that the common-mode voltages are generated by the
rectification and inversion process of the converters. This phenomenon
is different from the high dv/dt caused by the switching transients of
the high speed switches. It should be further noted that the common-mode
voltage issue is often ignored in the low voltage drives. This is partially
due to the conservative design of the insulation sys tem for low-voltage
motors. In the MV drives, the motor shouldn't be subject to any common-mode
voltages. Otherwise, the replacement of the damaged motor would be very
costly in addition to the loss of production.

(c) Motor De-rating. High-power inverters may generate a large amount
of current and voltage harmonics. These harmonics cause additional power
losses in the motor winding and magnetic core. As a consequence, the motor
is de-rated and cannot operate at its full capacity.

(d) LC Resonances. For the MV drives with a motor-side filter capacitor,
the capacitor forms an LC resonant circuit with the motor inductances.
The resonant mode of the LC circuit may be excited by the harmonic voltages
or currents produced by the inverter. Although the motor winding resistances
may provide some damping, this problem should be addressed at the design
stage of the drive.

(e) Torsional Vibration. Torsional vibrations may occur in the MV drive
due to the large inertias of the motor and its mechanical load. The drive
system may vary from a simple two-inertia system consisting of only the
motor and the load inertias to very complex systems such as a steel rolling-mill
drive with more than 20 inertias. The torsional vibrations may be excited
when the natural frequency of the mechanical system is coincident with
the frequency of torque pulsations caused by distorted motor currents.
Excessive torsional vibrations can result in broken shafts and couplings, and also cause damages to the other mechanical components in the system.

Switching Device Constraints

(a) Device Switching Frequency. The device switching loss accounts for
a significant amount of the total power loss in the MV drive. The switching
loss minimization can lead to a reduction in the operating cost when the
drive is commissioned. The physical size and manufacturing cost of the
drive can also be reduced due to the reduced cooling requirements for the
switching devices. The other reason for limiting the switching frequency
is related to the device thermal resistance that may prevent efficient
heat transfer from the device to its heatsink. In practice, the device
switching frequency is normally around 200 Hz for GTOs and 500 Hz for IGBTs and GCTs.

The reduction of switching frequency generally causes an increase in harmonic
distortion of the line- and motor-side waveforms of the drive. Efforts
should be made to minimize the waveform distortion with limited switching
frequencies.

(b) Series Connection. Switching devices in the MV drive are often connected
in series for medium-voltage operation. Since the series connected devices and their gate drivers may don't have identical static and dynamic characteristics,
they may not equally share the total voltage in the blocking mode or during
switching transients. A reliable voltage equalization scheme should be
implemented to protect the switching devices and enhance the system reliability.

Drive System Requirements

The general requirements for the MV drive system include high efficiency,
low manufacturing cost, small physical size, high reliability, effective
fault protection, easy installation, self-commissioning, and minimum downtime
for repairs. Some of the application-specific requirements include high
dynamic performance, regenerative braking capability, and four-quadrant
operation.

CONVERTER CONFIGURATIONS

Multi-pulse rectifiers are often employed in the MV drive to meet the
line-side harmonic requirements. ill. 3-1 illustrates a block diagram of
12-, 18- and 24 pulse rectifiers. Each multi-pulse rectifier is essentially
composed of a phase-shifting transformer with multiple secondary windings
feeding a set of identical six-pulse rectifiers.

Both diode and SCR devices can be used as switching devices. The multi-pulse
diode rectifiers are suitable for VSI-fed drives while the SCR rectifiers
are normally for CSI drives. Depending on the inverter configuration, the
outputs of the six pulse rectifiers can be either connected in series to
form a single dc supply or connected directly to a multilevel inverter
that requires isolated dc supplies. In addition to the diode and SCR rectifiers,
PWM rectifiers using IGBT or GCT devices can also be employed, where the
rectifier usually has the same topology as the inverter.

To meet the motor-side challenges, a variety of inverter topologies can
be adopted for the MV drive. ill. 3-2 illustrates per-phase diagram of
commonly used three-phase multilevel VSI topologies, which include a conventional
two-level inverter, a three-level neutral-point clamped (NPC) inverter,
a seven-level cascaded H-bridge inverter and a four-level flying-capacitor
inverter. Either IGBT or GCT can be employed in these inverters as a switching
device.

Current source inverter technology has been widely accepted in the drive
industry. ill. 3-3 shows the per-phase diagram of the CSI topologies for
the MV drive. The SCR-based load-commutated inverter (LCI) is specially
suitable for very large synchronous motor drives, while the PWM current
source inverter is a preferred choice for most industrial applications.
The parallel PWM CSI is composed of two or more single-bridge inverters
connected in parallel for super-high-power applications. Symmetrical GCTs
are normally used in the PWM current source inverters.

A number of MV drive products are available on the market today. These
drives come with different designs using various power converter topologies and control schemes. Each design offers some unique features but also has
some limitations.

The diversified offering promotes the advancement in the drive technology and the market competition as well. A few examples of the MV industrial
drives are as follows.

ill. 4-1 illustrates the picture of an MV drive rated at 4.16 kV and 1.2 MW.

The drive is composed of a 12-pulse diode rectifier as a front end and a three-level NPC inverter using GCT devices. The drive's digital controller
is installed in the left cabinet. The cabinet in the center houses the
diode rectifier and air-cooling sys tem of the drive. The inverter and its output filters are mounted in the right cabinet.

The phase-shifting transformer for the rectifier is normally installed
outside the drive cabinets.

The IGBT-heatsink assemblies in the central cabinet are constructed in
a modular fashion for easy assembly and replacement. The front end converter
is a standard 12-pulse diode rectifier for line current harmonic reduction.
The phase-shifting transformer for the rectifier isn't included in the
drive cabinet.

A 4.16-kV 7.5-MW cascaded H-bridge inverter-fed drive is illustrated in
Ill. 4-3. The inverter is composed of 15 identical IGBT power cells, each
of which can be slid out for quick repair or replacement. The waveform
of the inverter line to-line voltage is composed of 21 levels, leading
to near-sinusoidal waveforms without using LC filters. The drive employs
a 30-pulse diode rectifier powered by a phase-shifting transformer with
15 secondary windings. The transformer is installed in the left cabinets
to reduce the installation cost of the cables connecting its secondary
windings to the power cells.

ill. 4-4 shows a current source inverter-fed MV drive with a power range
from 2.3 MW to 7 MW. The drive comprises two identical PWM GCT current
source converters, one for the rectifier and the other for the inverter.
The converters are installed in the second cabinet from the left. The dc
inductor required by the cur rent source drive is mounted in the fourth
cabinet. The fifth (right most) cabinet contains drive's liquid cooling
system. With the use of a special integrated dc inductor having both differential- and common-mode inductances, the drive does not require an isolation transformer
for the common-mode voltage mitigation, leading to a reduction in manufacturing
cost.

Tbl 4-1 provides a summary of the MV drive products offered by major
drive manufacturers in the world, where the inverter configuration, switching
de vice, and power range of the drive are listed.